Also, if you don't want it drawing power you might as well take it out. It'll probably suck some from the motherboard otherwise...
PCI-Express cards can draw a lot of power from the motherboard. It's their primary
power source. Those connectors on the back are only auxiliary
. They're used because the power that can be drawn from the motherboard is limited: 75W in PCI-e 1.0, 150W in 2.0.
If the card's in the system and not used, it will still draw idle power. If it's in the system, not used, and not connected to auxiliary power, it will probably generate errors when its firmware detects that there's no auxiliary power.
So the only
two options are connect it fully, or take it out. If you leave it fully connected and not used, it will still draw only idle power.